Almost one in every fifty-five Americans have been exposed to the Hepatitis C Virus (HCV), but most are unaware of their infection because the virus causes few acute symptoms. If left untreated, the majority of HCV infections lead to chronic active hepatitis that eventually progresses to cirrhosis, cancer, or liver failure. Current therapies involving the drugs interferon and ribavirin are costly and produce debilitating side effects, frequently worse than the symptoms produced by HCV itself. Newer treatments are, however, quite effective against certain viral genotypes. This proposal will examine the HCV proteins most directly involved in viral replication, the NS3 Helicase and NS5B polymerase, as putative targets for the drug ribavirin and as targets for new antiviral agents. In addition to its established role as a modulator of the immune system, ribavirin has been proposed to eliminate viruses as a mutagen or through direct effects on viral replicative proteins. One popular hypothesis states that ribavirin's enhancement of the already high HCV mutation rate leads to a catastrophe of errors and subsequent virus elimination. Here, ribavirin effects will be examined in vitro, in enzyme assays, and in vivo, using a novel HCV replicon that should allow the assessment of replication fidelity. To attempt to relate ribavirin effects to genotype-specific drug response, all experiments will be repeated with the three most common American HCV genotypes, two that normally do not respond to therapy (la and lb) and one that frequently responds to therapy (2a). The polymerase and helicase proteins from each genotype will also be characterized to define conserved and divergent properties. A rigorous biochemical approach will be used to define enzyme differences because sequence data alone does not accurately predict protein structure or function. Preliminary data show that different genotypes encode enzymes with markedly different properties, hampering current rational drug design efforts. Structure-based site-directed mutagenesis will be used to determine the genetic basis for HCV enzyme variability. The biological consequences (i.e. replication rate, fidelity, protein expression) of HCV genetic variation will then be analyzed in a replicon system. The delineation of genetic variations responsible for certain phenotypes might allow the prediction of patient response to current or future HCV therapies, and the clear identification of conserved HCV enzyme properties will aid future HCV drug development.